In a typical cellular radio system, mobile terminals (also referred to as user equipment, UEs, wireless terminals, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks, which provide access to data networks, such as the Internet, and/or to the public-switched telecommunications network (PSTN). A RAN covers a geographical area that is divided into cell areas, with each cell area being served by a radio base station (also referred to as a base station, a RAN node, a “NodeB”, and/or an enhanced NodeB or “eNodeB”). A cell area is a geographical area over which radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with wireless terminals within range of the base stations.
Cellular communications system operators have begun offering mobile broadband data services based on, for example, WCDMA (Wideband Code-Division Multiple Access), HSPA (High-Speed Packet Access), and Long Term Evolution (LTE) wireless technologies. Fueled by the introduction of new devices designed for data applications, end user performance requirements continue to increase. The increased adoption of mobile broadband has resulted in significant growth in traffic handled by high-speed wireless data networks. Accordingly, techniques that allow cellular operators to manage networks more efficiently are desired.
Techniques to improve downlink performance may include Multiple-Input-Multiple-Output (MIMO) multi-antenna transmission techniques, multi-flow communication, multi-carrier deployment, etc. Since spectral efficiencies per link may be approaching theoretical limits, next steps may include improving spectral efficiencies per unit area. Further efficiencies for wireless networks may be achieved, for example, by changing a topology of traditional networks to provide increased uniformity of user experiences throughout a cell. One approach is through the deployment of so-called heterogeneous networks.
A homogeneous network is a network of base stations (also referred to as NodeBs, enhanced NodeBs, or eNBs) in a planned layout, providing communications services for a collection of user terminals (also referred to as user equipment nodes, UEs, and/or wireless terminals), in which all base stations typically have similar transmit power levels, antenna patterns, receiver noise floors, and/or backhaul connectivity to the data network. Moreover, all base stations in a homogeneous network may generally offer unrestricted access to user terminals in the network, and each base station may serve roughly a same number of user terminals. Current cellular wireless communications systems in this category may include, for example, GSM (Global System for Mobile communication), WCDMA, HSDPA (High Speed Downlink Packet Access), LTE (Long Term Evolution), WiMAX (Worldwide Interoperability for Microwave Access), etc.
In a heterogeneous network, low power base stations (also referred to as low power nodes (LPNs), micro nodes, pico nodes, femto nodes, relay nodes, remote radio unit nodes, RRU nodes, small cells, RRUs, etc.) may be deployed along with or as an overlay to planned and/or regularly placed macro base stations. A macro base station (MBS) may thus provide service over a relatively large macro cell area, and each LPN may provide service for a respective relatively small LPN cell area within the relatively large macro cell area.
Power transmitted by an LPN may be relatively small, e.g., 2 Watts, compared to power transmitted by a macro base station, which may be 40 Watts for a typical macro base station. An LPN may be deployed, for example, to reduce/eliminate a coverage hole(s) in the coverage provided by the macro base stations, and/or to off-load traffic from macro base stations, such as to increase capacity in a high traffic location or so-called hot-spot. Due to its lower transmit power and smaller physical size, an LPN may offer greater flexibility for site acquisition.
Thus, a heterogeneous network features a multi-layered deployment of high-power nodes (HPNs), such as macro base stations, and low-power nodes (LPNs), such as so-called pico-base stations or pico-nodes. The LPNs and HPNs in a given region of a heterogeneous network may operate on the same frequency, in which case the deployment may be referred to as a co-channel heterogeneous deployment, or on different frequencies, in which case the deployment may be referred to as an inter-frequency or multi-carrier or multi-frequency heterogeneous deployment.
The Third Generation Partnership Project (3GPP) is continuing to develop specifications for advanced and improved features in the context of the fourth-generation wireless telecommunications system known as LTE (Long Term Evolution). In Release 12 of the LTE specifications and beyond, further enhancements related to low-power nodes and heterogeneous deployments will be considered under the umbrella of “small-cell enhancements” activities. Some of these activities will focus on achieving an even higher degree of interworking between the macro and low-power layers, including through the use of a set of techniques and technology referred to as “dual-layer connectivity” or simply “dual connectivity.”
As shown in FIG. 1, dual connectivity implies that the device has simultaneous connections to both macro and low-power layers. FIG. 1 illustrates an example of a heterogeneous network in which a mobile terminal 101 uses multiple flows, e.g., an anchor flow from the macro base station (or “anchor eNB”) 401A and an assisting flow from a pico base station (or an “assisting eNB”) 401B. Note that terminology may vary—the anchor base station and assisting base station in a configuration like that shown in FIG. 1 may sometimes be referred to as “master” and “slave” base stations or according to other names. It should be further noted that while the terms “anchor/assisting” and “master/slave” suggest a hierarchical relationship between the base stations involved in a dual connectivity scenario, many of the principles and techniques associated with dual connectivity may be applied to deployment scenarios where there is no such hierarchical relationship, e.g., between peer base stations. Accordingly, while the terms “anchor base station” and “assisting base station” are used herein, it should be understood that the techniques and apparatus described herein are not limited to embodiments that use that terminology, nor are they necessarily limited to embodiments having the hierarchical relationship suggested by FIG. 1.
Dual connectivity may imply, in various embodiments and/or scenarios:                Control and data separation where, for instance, the control signaling for mobility is provided via the macro layer at the same time as high-speed data connectivity is provided via the low-power layer.        A separation between downlink and uplink, where downlink and uplink connectivity is provided via different layers.        Diversity for control signaling, where Radio Resource Control (RRC) signaling may be provided via multiple links, further enhancing mobility performance.        
Macro assistance including dual connectivity may provide several benefits:                Enhanced support for mobility—by maintaining the mobility anchor point in the macro layer, as described above, it is possible to maintain seamless mobility between macro and low-power layers, as well as between low-power nodes.        Low overhead transmissions from the low-power layer—by transmitting only information required for individual user experience, it is possible to avoid overhead coming from supporting idle-mode mobility within the local-area layer, for example.        Energy-efficient load balancing—by turning off the low-power nodes when there is no ongoing data transmission, it is possible to reduce the energy consumption of the low-power layer.        Per-link optimization—by selecting the termination point for uplink and downlink separately, the node selection can be optimized for each link.        
One of the problems in using dual connectivity is how to map the data radio bearers (DRBs) onto the anchor flow and assisting flow, respectively. One option for splitting the DRBs between two base stations, as shown in FIG. 1, is to keep the control plane (RRC) in the anchor eNB and distribute the PDCP entities so that some of them are in the anchor eNB and some of them in the assisting eNB. As discussed in further detail below, this approach may yield some important system efficiency benefits. However, this approach creates problems related to the handling of security keys that are used for confidentiality and integrity protection of the data transmitted to and from the mobile terminal.